1. Field of the Invention
The present invention relates to a video data reading apparatus using a USB (universal serial bus) interface such as a video data reading apparatus of a PC camera for successively reading a moving picture and transmitting the moving picture via a USB interface to a personal computer.
2. Description of the Related Art
The USB interface was proposed in 1996 and began to be loaded on a personal computer in 1997. The USB interface itself is a new technique and there has been no video data reading apparatus using the USB interface.
It is possible to consider a PC camera employing the conventional PC camera technique and the USB specification as a PC camera having: video input means for inputting a video data; key input means for entering a key-in data; and data output means for transmitting the video data and the key-in data via a USB interface to a personal computer.
In this PC camera, a key-in data end point is provided separately from a video data end point so that when a key switch is depressed, the key-in data end point is used so as to transmit the key-in data without synchronization with the video data.
However, such a configuration has a problem as follows.
(1) To provide an end point in a USB interface logic circuit mounted on a PC camera, it is necessary to provide a large-scale circuit having a number of registers and gates such as FIFO. If another end point is to be provided for a key-in data transmission, the PC camera circuit size is further increased, which in turn increases production costs.
(2) Each end point data transfer requires a predetermined transmission time according to the USB specification including an overhead. Moreover, a key-in data is transferred in a packet separately from a video data transfer by way of interrupt transfer or isochronous transfer (hereinafter, referred to as ISO transfer) defined in the USB specification. When transmitting this key-in data, it is necessary to use a predetermined band width separately from the video data transfer, including the overhead required for transmission. Thus, while transmitting a key-in data, it is impossible to transfer a video data. This significantly lowers the video data transfer rate.
As has been described above, according to the prior art, there are problems that (1) the PC camera circuit configuration becomes complicated and increases its size and (2) the video data transfer rate is lowered when transmitting a key-in data.